The present invention is directed to a method for producing phosgene. More particularly the invention relates to a method for producing phosgene by reaction of carbon monoxide and chlorine in the presence of at least one catalyst.
Phosgene (sometimes known as carbonyl chloride or carbonyl dichloride) finds use in the preparation of organic compounds, monomers and polymers, such as carbonates, isocyanates, chloroformates, carbamates, polyurethanes, and polycarbonates. Various methods for producing phosgene have been previously described in the literature and utilized by industry. One method that is frequently used involves reaction of carbon monoxide with chlorine in the presence of a carbon-comprising catalyst such as activated carbon. The reaction is strongly exothermic and is usually done in multitubular reactors to more effectively control the reaction temperature. Phosgene produced by this method typically contains carbon tetrachloride as a byproduct, often as much as about 250-300 ppm or higher by volume, which may be difficult to separate from phosgene product. For some applications the level of carbon tetrachloride in phosgene often must be removed to a level of about 1 ppm before use of the phosgene. Carbon tetrachloride arises from reaction of chlorine with carbon-comprising catalyst and thus results directly in depletion of catalyst. Carbon tetrachloride has also been implicated in both ozone depletion and in global warming. On a global basis the amount of byproduct carbon tetrachloride produced in commercial phosgene manufacture annually may be as much as 2 million kilograms based on phosgene production of about 4 billion kilograms. Therefore, methods continue to be sought for minimization of carbon tetrachloride byproduct formation using carbon catalysts while maintaining as high a rate of formation of phosgene as possible.
Japanese patent publication No. 02[1990]-06,307 discloses that the amount of carbon tetrachloride produced during a phosgene manufacturing process can be reduced by about 50% to about 150 ppm by using an activated carbon which has been washed with an acid and which contains a total of about 1.5 wt. % or less of metal components comprised of transition metals, boron, aluminum and silicon.
After diligent experimentation the present inventors have discovered a method for substantially reducing the amount of carbon tetrachloride byproduct produced in phosgene manufacture while maintaining a high rate of formation of phosgene. In one of its embodiments the present invention is a method for producing phosgene which comprises contacting in at least one reactor a mixture comprising carbon monoxide and chlorine sequentially with a first catalyst followed by contacting the resulting gaseous reaction mixture comprising phosgene with at least one second catalyst of higher relative activity than a first catalyst. In another of its embodiments the present invention is a method for producing phosgene which comprises contacting in at least one reactor a mixture comprising carbon monoxide and chlorine with at least one catalyst bed comprising a first catalyst wherein at least a portion of said first catalyst is diluted with a second catalyst of higher relative activity than a first catalyst. Various other features, aspects, and advantages of the present invention will become more apparent with reference to the following description and appended claims.